In the state of the art, a known method of soldering an element, such as a chip, onto a substrate by providing a solder-forming mass, comprises:                forming a vertical stack of axis coinciding substantially with the gravity direction and comprising, from top to bottom: the element; the mass in the solid state; and the substrate; and        heating the mass so as to cause it to pass into a liquid state enabling it to spread on the substrate.        
The quality of the soldering depends in particular on the wettability properties of the surface of the substrate that ensure good wetting of the substrate by the mass, i.e. that it spreads sufficiently. More precisely, the wettability of the surface is defined by a contact angle, also known as the wetting angle, that is formed between the tangent to the surface of the mass and the tangent to the surface of the substrate at the point of contact between the mass and the substrate.
In order to ensure good wetting of the mass on the substrate, it is desirable to increase the duration of the period during which the mass is in its liquid state, e.g. by lengthening the duration over which the mass is heated in order to ensure that it spreads sufficiently over the substrate.
Unfortunately, although the duration of heating can be determined relatively easily, it is difficult to determine accurately the duration of the period during which the mass was indeed in its liquid state.
By way of example, the mass may be heated by irradiating it with a continuous laser source that is controlled to produce long pulses, e.g. of a duration greater than about 2 seconds (s).
Under such circumstances, the length of time heat propagates through the substrate and the mass is no longer negligible compared with the duration of a laser pulse that corresponds to the duration of the heat being applied.
Consequently, the duration of the period during which the mass is actually in its liquid state does not necessarily correspond to the duration of the heat being applied.